In many systems, it is desirable to provide circuit packs which can become operationable in the event of a failure in the operable circuit packs. For example, in subscriber loop carrier transmission systems, it is common to include two different circuit packs comprising different Time Slot Interchangers (TSIs), one handling communications signals and one acting as a spare or alternate. It is important that the peripheral devices in such systems know which of the two TSIs is active. In order to keep the peripherals apprised of their status, the circuit packs including the TSIs send out signals through an exclusive OR gate so that if both packs send the same signal, a first one of the packs is active, and if the packs send out different signals, a second one of the packs is active.
While such sparing arrangements are generally effective, a problem arises when one of the circuit packs must be physically removed from the system, physically inserted, or there is a failure of the board mounted power supply on the circuit pack. In most cases, the signal normally originating from the removed, inserted or failed circuit pack slot to the peripherals would be changed, thus causing confusion among the peripherals as to which pack was active. It would be more desirable to hold the signal of a circuit pack slot at the level which existed just prior to the circuit pack removal, insertion, or power failure.